HOLMES. [VoL. II. 



differences are produced by the variation of five elements : 

 (i), the color of the chitinous integument; (2), the color of 

 the blood and tissues ; (3), the contents of the alimentary canal ; 

 (4), the color of the sex glands; (5), the pigment cells. The 

 first of these factors is, perhaps, the least important and is not 

 subject to great variation. The exoskeleton over most of the 

 surface of the body is colorless ; on the antennae it is marked 

 with transverse reddish-brown bands which give the light red- 

 dish-brown annulations of these organs. This color is seen as 

 distinctly in the shed skin as in the living animal. 



The color of the tissues and blood is subject to great varia- 

 tion. The green color of Amphithoe, or the blue green tint 

 when it occurs, is due to some coloring matter that is uniformly 

 diffused throughout the animal. In some specimens there is a 

 sufficient amount present to give the animal a brilliant emerald 

 green, but many may be found in which not the slightest trace 

 of green coloration could be detected. This green color may 

 be seen to undergo marked changes in intensity if individuals 

 be watched for several days. The blue color is much rarer. 

 One specimen in which this blue coloration was strongly 

 marked was kept under observation for several days. After 

 five days most of the blue color had disappeared, the green 

 becoming more nearly like the typical green of other forms. 

 After six days the green was not to be distinguished from 

 the ordinary type ; the green color then gradually became 

 fainter, and on the ninth day the tissues were whitish, scarcely 

 a trace of green being visible. During all this time the speci- 

 men ate abundantly of green algae, judging from the amount of 

 excrement consisting of Ulva cells that accumulated in the dish. 



The contents of the alimentary canal influence to a consider- 

 able extent the general color effect produced by the animal 

 when seen by the naked eye. If they consist largely of green 

 Ulva, they tend to give the animal a greenish appearance. If 

 the Ulva has been subjected some time to the action of the 

 digestive juices and become a yellowish color, it tends to give 

 the animal a corresponding yellowish aspect. Light becoming 

 colored by passing through the alimentary canal is reflected 

 and re-reflected in the tissues and tends to make them appear 



